1. Field of the Invention
The present invention relates to an optical pick-up apparatus which optically records information into an optical recording medium and/or reproduces information from an optical recording medium.
2. Description of the Related Art
Optical disks like Compact Disc (CD), Digital Versatile Disc (DVD), and Mini Disc (MD), etc. are utilized as optical recording media in various fields such as audio video and computer, etc. In accordance with increasing demand of storage capacity, i.e. information volume to be recorded into above mentioned optical recording media, also getting along with narrower track pitch which is a track interval formed on the optical recording medium, information storing area has been expanding to nearly center inside of the optical recording medium.
In an information reproducing apparatus using such an optical recording medium, information are recorded or reproduced by converging a light spot onto information recording surface of the optical recording medium, and by tracking light spots on a track formed in the optical recording medium. A control which tracks light spots on the track is called a tracking control. The tracking control is carried out by detecting a light reflected from the optical recording medium through light receiving elements, and by performing feedback a detected signal from the light receiving elements to an actuator driving an objective lens which is light converging means for converging light onto the optical recording medium. A signal which is used for performing feedback control of driving of the actuator is called a tracking error signal (abbreviated to “TES” hereafter). There is a differential push pull (DPP) method as one of signal generating methods using the tracking error signal.
The DPP method is disclosed in Japanese Unexamined Patent Publication JP-A 7-93764 (1995) for example. FIG. 16 is a simplified schematic diagram showing a structure of a conventional optical pick-up apparatus 1 using the DPP method. An example of a structure of the conventional optical pick-up apparatus 1 is as follows. The optical pick-up apparatus 1 comprises a semiconductor laser 2 as a light source, a collimator lens 3, a diffraction grating 4, a beam splitter 5, a quarter-wavelength plate 6, an objective lens 7, a condensing lens 8, and a photo-detector 9 formed of a light receiving element.
In the optical pick-up apparatus 1, light irradiated from the semiconductor laser 2 is changed to substantially parallel light through the collimator lens 3, diffracted into at least zero-order diffraction light, plus (+) first-order diffraction light and minus (−) first-order diffraction light through the diffraction grating 4, then, transmits the beam splitter 5, transformed into a circularly-polarized light through the quarter-wavelength plate 6, and irradiated onto an optical recording medium 10 after converging light through the objective lens 7.
FIGS. 17A and 17B are views showing a condition of zero-order diffraction light and ±first-order diffraction light which are irradiated on the optical recording medium 10. FIG. 17A shows locations of zero-order diffraction light, + and − (±) first-order diffraction lights irradiated onto the track formed in the optical recording medium 10, and FIG. 17B shows a cross-sectional shape of the optical recording medium 10. The tracking control is carried out so as to irradiate a center of a land part 11 of the track in a width direction in which information should be recorded or which information to be reproduced is recorded (hereinafter maybe the land part 11 is called information track), with a main beam (hereinafter abbreviated to “MB”) comprising the zero-order diffraction light. In this occasion, locations which are out of alignment by one half track pitch each toward groove parts 12 and 13 adjacent to both sides of the information track 11 irradiated with the MB are irradiated with a first sub beam (hereinafter abbreviated to “SB1”) which is + first-order diffraction light and a second sub beam (hereinafter abbreviated to “SB2”) which is − first-order diffraction light.
The MB, SB1, and SB2 with which the optical recording medium 10 is irradiated are reflected by the optical recording medium 10 and again transmitted through the objective lens 7 and the quarter-wavelength plate 6, then reflected by the beam splitter 5, condensed through the condensing lens 8 and received onto the photo-detector 9.
FIG. 18 shows a schematic circuit diagram for obtaining a DPP signal based on detecting signals from the photo-detector 9. The photo-detector 9 comprises photo-detectors 9b, 9c formed of a light receiving element which is divided in two parts so as to have a parting line in a direction parallel to a direction in which a track formed in the optical recording medium 10 extends, and a photo-detector 9a including a light receiving element which divided into quarters so as to have parting lines in a directions parallel to and perpendicular to the track extending direction, under conditions where the optical recording medium 10 is attached facing the optical pick-up apparatus 1.
When an MB push pull signal obtained from an MB receiving light signal detected by the photo-detector 9a and a subtracter 14 is defined as an MPP (Main Push Pull), an SB1 push pull signal obtained from an SB1 receiving light signal detected by the photo-detector 9b and a subtracter 15 is defined as an SPP1 (Sub Push Pull-1), an SB2 push pull signal obtained from an SB2 receiving light signal detected by the photo-detector 9c and a subtracter 16 is defined as an SPP2 (Sub Push Pull-2), a DPP signal which is calculated from a subtracter 19 based on an amplified signal which is further amplified an addition signal SPP (=SPP1+SPP2) obtained from the SPP1, the SPP2 and an accumulator 17, with an amplifier 18 and the MPP is obtained according to the following formula (1).DPP=MPP−k (SPP1+SPP2)  (1)
Where a gain k with the amplifier 18 is a coefficient used to compensate the difference of light intensity of zero-order diffraction light and ± first-order diffraction light. When light intensity ratio of each diffraction light is as follows:light intensity of zero-order diffraction light: light intensity of + first-order diffraction light:light intensity of − first-order diffraction light=a:b:b, k is obtained from k=a/(2b).
As above mentioned, locations which are out of alignment by one half track pitch adjacent to both sides from information track 11 irradiated with the MB, are irradiated with the SB1 and the SB2. Therefore, phases of the SPP1 and the SPP2 are out of alignment by 180 degrees against a phase of the MPP, respectively. FIG. 19 is a view showing one example of push pull signals. In FIG. 19, an example is shown in a case where light intensities of the above mentioned diffraction lights are equal to each other, and a=b, i.e. k=0.5. Since light intensities of the SPP1 and the SPP2 are equal to each other, the SPP1 and the SPP2 overlap. Further, since the SPP which multiplies the sum of the SPP1 and the SPP2 by 0.5 is identical with the SPP1 and the SPP2, the SPP1 and the SPP2 overlap. Since the phases of the MPP and the SPP are reverse phases shifted by 180 degrees, the DPP signal is obtained from adding absolute values of amplitudes of the MPP and the SPP.
FIG. 20 is a view showing one example of push pull signals in condition of generating an offset ΔP. Even in a condition where a predetermined track position of the optical recording medium 10 is irradiated with each diffraction light, there is a case where the offset ΔP generates due to a shift of the objective lens or a tilt of the optical recording medium 10. However, even in case where such an offset ΔP generates, since the MPP and the SPP are reverse phases as aforementioned, it is possible to obtain the DPP signal in which the offset ΔP is canceled according to a calculation based on the formula (1).
However, in the conventional DPP method disclosed in JP-A 7-93764, there is a problem that accurate rotation adjustment of a relative position of the diffraction grating 4 with respect to the optical recording medium 10 must be carried out so as to dispose the SB1 and the SB2 to shift by ½ track pitch exactly with respect to the MB. Further, in the conventional DPP method disclosed in JP-A 7-93764, an effect from a track curvature formed in the optical recording medium 10 shown in the above mentioned FIG. 17 is not considered.
FIG. 21 is a view showing a condition of zero-order diffraction light and ± first-order diffraction light with which an optical recording medium 21 is irradiated, with considering a track curvature. As mentioned above, in accordance with increasing demand of storage capacity, nearly to the center inside of the optical recording medium 21 is utilized for information storing and reproducing. Therefore, when signals from the track formed around center area of the optical recording medium 21 is detected, the effect of the track curvature must be considered.
As shown in FIG. 21, when a curvature exists in the track, in case of disposing the SB1, which is a leading beam of the sub beams, into a center of a track groove part 23, it is impossible to dispose the SB2, which is a following beam, into a center of a track groove part 24. In a servo control method using three beams (MB, SB1, SB2) from zero-order diffraction light and ± first-order diffraction lights, it is a common way to dispose SB1 and SB2 on the track groove parts 23 and 24 which are adjacent to both sides of a information track 22 (land part) on which the MB is disposed, respectively. However, there is a problem, as shown in FIG. 21, when a track curvature exists, in case of disposing the MB on the center of the information track 22, that it is impossible to dispose the SB1 in the center of the groove part 23 adjacent to the information track 22, and at the same time to dispose the SB2 in the center of the groove part 24.
FIG. 22 is a view showing an example of a DPP signal obtained based on the detecting signals of the MB, the SB1 and the SB2 with which the track having a track curvature is irradiated. As shown in FIG. 21, in case of an impossibility to dispose the MB on the information track 22, at the same time disposing the SB1 and the SB2 onto the center of the groove part 23 and 24 adjacent to the information track 22, the SPP signal which is a sum signal of the SPP1 and the SPP2 against the MPP has a phase difference. Therefore, another phase difference occurs between the MPP and the DPP which is obtained from the formula (1), and this phase difference becomes a track offset.
On the contrary, in case of disposing the SB2 in the center of the groove part 24, a track offset occurs due to an impossibility of disposing the SB1 in the center of the groove part 23. Further, there is a method for reducing an offset by inverting the phases of the sub beams, i.e., the SB1 and the SB2 against the phase of the MB, and disposing the three beams onto the same track, by contriving a structure of the diffraction grating. However, since there is an isolation distance between the leading and following beams, it is difficult to arrange all of the three beams in the center of the same track when a track curvature exists, and a track offset occurs in the end.
Like this way, when a track curvature exists, even a rotation adjustment for relative positioning against the optical recording medium 21 of the diffraction grating 4 is carried out strictly, it is impossible to eliminate the phase difference between the sum signal SPP of the SB1 and the SB2 and the push pull signal MPP of the MB, and there is a problem that a track offset remains.
Another conventional technique to solve such a problem is disclosed in Japanese Unexamined Patent Publication JP-A 2001-250250 (2001). The description of the technique disclosed in JP-A 2001-250250 is as follows.
FIG. 23 is a simplified schematic diagram showing a structure of the optical pick-up apparatus 25 which is applied with another conventional technique. The optical pick-up apparatus 25 comprises a semiconductor laser 26, a collimator lens 27, a diffraction grating 28, a beam splitter 29, an objective lens 30, a condensing lens 31, and an optical power detector 32 (simplified view) having the same structure with the optical power detector 9. Here, the definitions about X, Y, and Z axes which are three-dimensional coordinate axes shown in FIG. 23. FIG. 24 is a schematic plan view of an optical recording medium 33 from the side of light beams are condensed. Z axis is an axis in an axial direction of light which is irradiated from the semiconductor laser 26 and condensed on an information recording surface of the optical recording medium 33. X axis is an axis provided in an extension direction of a segment 36 which connects the center 34 of the optical recording medium 33 and a focusing position 35 where the light irradiated from the semiconductor laser 26 is focused on the information recording surface of the optical recording medium 33, in a virtual plane perpendicular to Z axis. Accordingly, the X axis direction is called a radial direction, since this axis corresponds to the radial direction of the optical recording medium 33. Y axis is an axis extending in direction perpendicular to X axis, in the virtual plane perpendicular to Z axis. Accordingly, there is an occasion that the Y axis direction is called a track direction, since this axis corresponds to a tangential direction of a track formed in the optical recording medium 33. These definitions according to three axis directions are used in the specification in common.
FIG. 25 is a top plan view showing a structure of the diffraction grating 28 (patterning) provided in the conventional optical pick-up apparatus 25. In the diffraction grating 28, a quartered area part 37 which is obtained in case of cutting into quarters equally by parting lines which are parallel to X and Y axis directions respectively, is formed so as to be different from the remaining area part 38 other than the quartered area part 37. In the diffraction grating 28 shown in FIG. 25, the quarter area 37 is formed in a lower right corner on top plan view. The quarter area part 37 and the remaining part 38 are structured in uniform with respect to a grating groove direction and a grating groove interval. However, since pitches between the grating groove intervals are located out of alignment only one half each other, in a phase of a light which transmits the quarter area part 37, 180 degrees phase difference is added in relation to a phase of a light which transmits the area part 38 except for the quarter part.
FIG. 26 is a view showing the TES in case of using the diffraction grating 28. In a case where a light beam 39, transmits the diffraction grating 28, is diffracted into the MB, the SB1 and the SB2, further reflected by the optical recording medium 33, and received by the optical power detector 32, the area of the diffraction light having 180 degrees phase difference against the beam irradiating area is substantially equal to the area of the diffraction light which has no connection with the phase difference against the beam irradiating area with respect to the light receiving element receiving the SB1 and the SB2, i.e. ± first-order diffraction lights, i.e., a light receiving amount of the diffraction light having the phase difference is substantially equal to that of the diffraction light having no connection with the phase difference. Therefore, the diffraction lights having 180 degrees phase difference with respect to each other are canceled, and the amplitudes of the SPP1 and the SPP2 of the push pull signals from the sub beams SB1 and SB2 will become substantially zero.
In the meantime, with respect to the offset occurring due to the shift of the objective lens 30, it is possible to cancel in accordance with the calculation of the above mentioned formula (1). As described above, in the other conventional optical pick-up apparatus 25, by making the amplitudes of the SPP1 and the SPP2 of the push pull signals from the SB1 and the SB2 substantially zero, the reduction of the track offset occurring due to a rotation adjustment error of the diffraction grating 28 is realized.
However, in the other conventional optical pick-up apparatus 25, when the quarter area part 37 which adds 180 degrees phase difference of the diffraction grating 28 generates a deviation in X and/or Y axis direction which is perpendicular to the optical axial direction against the light beam 39, a phenomenon occurs that an amplitude ratio increases according to increase of deviation amount. Here, the amplitude ratio means a ratio of an amplitude of the push pull signal SPP1, SPP2 by the SB1 or the SB2 against an amplitude of the push pull signal MPP by the MB (i.e., SPP1/MPP or SPP2/MPP).
FIG. 27 is a view showing a relationship between the deviation amount and the amplitude amount. In FIG. 27, the amplitude ratio of the amplitude of the SPP1 against the amplitude of the MPP (SPP1/MPP) is exemplified. In FIG. 27, line 40 shows the change of amplitude ratio in case of occurring the deviation in X axis (radial) direction, and line 41 shows the change amplitude ratio in case of occurring the deviation in Y axis (track) direction. With respect to the diffraction grating 28 having the patterning shown in the FIG. 25, in case of occurring the deviation in Y axis direction, an increasing sensitivity to the amplitude ratio is higher than a case of occurring the deviation in X axis direction.
Accordingly, in the other conventional optical pick-up apparatus 25, there is a problem that the track offset generates when the deviation with a rotation position adjustment of the diffraction grating 28 occurs in a condition where the phase difference addition area part 37 causing the deviation in X axis direction and/or Y axis direction so that the amplitude ratio increases, i.e., in a condition where the amplitude of push pull signals SPP1 and SPP2 from the SB1 and the SB2 are large. In other words, in the optical pick-up apparatus 25 using the diffraction grating 28 which adds a phase difference to part of the light beam 39, there is also a problem that high precision is required with respect to the rotation position adjustment of the diffraction grating 28.
Further, generally, in the optical pick-up apparatus, the diffraction grating is attached to a cylindrical holder having rotating axis, because the rotation adjustment of the diffraction grating must be carried out in order to arrange the MB, the SB1 and the SB2 into the target positions on the optical recording medium. When the holder is attached to the diffraction grating, in a case where an error generates between the center of the diffraction grating and the rotating axis of the holder, this condition equally corresponds to a condition where the deviation in X axis direction and/or Y axis direction occurs at an area part which adds the phase difference to the light beam. Therefore, when the rotation position of the diffraction grating is adjusted, the amplitudes of push pull signals SPP1, SPP2 by the SB1 and the SB2 increase, which causes a track offset occurrence.